It is known to use diffractometry to detect certain crystalline substances such as most explosives or numerous other dangerous or illegal structures. Within a crystal, which is an arrangement of atoms, elastically scattered electromagnetic waves interfere with each other to give scattering which is coherent at the scale of the crystal. When those interferences are constructive, they may be detected by the measurement of a diffracted ray and by the identification of the diffraction peaks. Thus, the constructive interferences are located by an appearance of diffraction peaks (or Bragg peaks) in the radiation diffused by a material.
To know whether a given crystalline substance is contained in a material, it is thus known to:
irradiate a sample of the material using an incident beam with a central axis X, emitted by a source, and to study the diffracted radiation using a detection device comprising a detector, termed spectrometric detector, adapted to establish an energy spectrum of the radiation diffused at a given scatter angle, that is to say a detector comprising                a detector material, which, on the near side to the sample of material, presents a plane termed detection plane, and        
means, termed spectrometry measurement means, adapted to measure an energy released by each interaction of a photon with the detector material and to establish at least one energy spectrum, denoted S(E). A collimator, termed detection collimator, associated with the detector, the detector and the detection collimator being arranged so as to have a detection axis D, the detection axis D forming a diffraction angle θ with the central axis X of the incident beam.
It is to be noted that an energy spectrum illustrates the energy distribution of radiation in the form of a histogram representing the number of photon interactions in the material (along the y-axis) according to the released energy (along the x-axis). Generally, the energy axis is discretized into channels of width 2 δE, a channel Ci corresponding to the energies comprised between Ei−δE and Ei+δE.
The various peaks obtained on an energy spectrum of a radiation that is scattered, at an angle θ, are characteristic of the material analyzed, since the scattered radiation participating in the constructive interferences satisfies the following equation:
      E    hkl    =      n    ⁢                  ⁢          hc              2        ⁢                  d          hkl                ⁢                  sin          ⁡                      (                          θ              /              2                        )                              
with:
dhkl: interplanar spacing between the crystallographic planes of the irradiated crystal;
θ: scatter angle, that is to say the angle formed between the scattered ray analyzed and the beam that is incident on the irradiated crystal
h: Planck's constant
c: the speed of light
n: the order of the interference.
This property is exploited in well-known methods, designated by the acronym EDXRD or “Energy Dispersive X-Ray Diffraction”.
WO2008/142446 describes a method of determining the composition of an object by the spectrometric detection of an object irradiated by x-ray radiation. In the description of the prior art of WO2008/142446, reference is made to baggage checking. The method described comprises the following steps:
irradiating the object, particularly for example using x-ray radiation,
detecting the intensity transmitted through the object using a spectrometric detector. It is to be noted that the radiation studied here is the radiation transmitted by the sample of material and is not diffracted; in other words, the detection axis D coincides with the axis X of the incident beam,
selecting energy bands in the transmitted spectrum, and establishing transmission quantities in each of those bands, and
comparing at least two of said obtained quantities.
According to a first embodiment, it is sought to identify the material by detecting a Bragg detection signature. For this, a discontinuity in the transmitted intensity is revealed, at a given energy, or at least within a narrow energy band. This discontinuity is assumed to correspond to a localized drop in the amplitude of the transmitted signal under the effect of elastic scattering (Bragg diffraction) in the crystal lattice of the material analyzed. This scattering only occurs for certain discrete incident energies Ei and it is considered that in the neighborhood of that energy, the transmitted signal decreases. Thus, by comparing the intensity of the signal transmitted at that energy in a narrow energy band centered on Ei with the signal transmitted at another energy, the presence of a particular material is detected. In other words, in this application, analysis is made of the radiation transmitted by the object, and in particular of the discontinuities in its energy spectrum on account of Bragg diffraction,
US20060140340 describes a device for identifying illicit or dangerous substances, comprising:                an x-ray source (2) associated with a primary collimator (3) delivering a beam of rectangular cross-section striking an inspection volume (6),        an energy resolving energy detector (11) associated with a secondary collimator (10), which energy detector (11) provides an energy spectrum of the radiation diffracted at an angle θ by the inspection volume,        a spatially resolving detector (13), which measures the intensity of the transmitted radiation, and        a conveyor belt 8.        
FIG. 4a of US20060140340 illustrates a first simulation with a sufficiently long acquisition time to give a good signal-to-noise ratio, but too long to envision the use of the method for detecting dangerous or illicit products in baggage. A second simulation carried out with a shorter acquisition time (compatible with the envisioned application, i.e. the detection of dangerous or illicit products in baggage) provides a signal of which the signal-to-noise ratio is low and leads to FIGS. 6a to 6c of US20060140340 being obtained, which are not possible to exploit.
To solve this problem, US20060140340 instructs to acquire diffraction data at a plurality of diffraction angles, either by moving the energy detector along a rail (12) in an arc of a circle centered on the source, or by using a plurality of detectors disposed at different diffraction angles. Each spectrum obtained is first of all corrected using a correction derived from a dual energy detection method. Next, the range of energies in each spectrum is divided into a plurality of narrower energy ranges. Each narrow energy range corresponds to a particular value of the momentum transfer. A diffraction model is then obtained by adding, point by point, the data substantially corresponding to the same momentum transfer values in the different spectra. The diffraction models so obtained for the two simulations (long and short acquisition time) are represented in FIGS. 5b and 7b, both of which can be exploited.
The first solution proposed by US20060140340, consisting of moving the detector along a circular rail, has the negative consequence of multiplying the processing time by the number of acquisitions carried out, which is not desirable for the application concerned. The second solution (increasing the number of detectors) leads to a bulky and costly diffractometer.
The technique of analysis by diffractometry, and more generally by spectrometry, requires the employment of a spectrometric detector that is sufficient energy resolving to enable the separation and the identification of the different characteristic peaks linked to the crystalline materials of the sample.
Generally, a Germanium type detector is used. This type of detector provides very attractive energy resolution but it must be cooled to very low temperatures, by complex and/or costly methods (thermoelectric cooling or cooling by a tank of liquid nitrogen). Also, the analysis devices employing such a detector are very bulky.
The recent emergence of spectrometric detectors capable of being used at ambient temperature, such as detector types implementing CdTe, CdZnTe, or scintillator materials (older technologies), provides an attractive alternative to the Germanium detectors. To be precise, these detectors are compact, not cooled and less costly. However, their performance in terms of energy resolution is still less than that obtained with the Germanium detectors.